The present invention relates to a color CRT (Cathode Ray Tube) and, more particularly, to a shadow mask for a color CRT to produce a display which is free from the local omission of stripes ascribable to the short exposure of the mask's outermost slots, and a method for producing the same.
A shadow mask for a color CRT has a number of rectangular slots formed therein for allowing electron beams to pass therethrough. Each slot consists of a rectangular large hole and a rectangular small hole communicated to each other and respectively formed on the front and rear of the mask. Generally, at the center of the mask, the center of each large hole and that of the small hole associated therewith are coincident with each other. On the other hand, towards the edge of the mask, the centers of the small holes are each deviated from the centers of the associated large holes, the centers of the small holes being positioned closer towards the center of the mask than the centers of the large holes. The deviation of the centers of the large and small holes sequentially increases from the centers towards the edge of the mask. With this configuration, the mask minimizes the quantity of, among exposing light having an angle of incidence, light directly incident to the outermost slots and undesirably reflected by the walls of such slots.
To better understand the present invention, brief reference will be made to the basic structure of a conventional color CRT, shown in FIG. 1. As shown, the color CRT, generally 10, is implemented as a bulb 12 having a face panel 14 at its front end. A fluorescent surface 16 and a shadow mask 18 are sequentially stacked on the inner surface of the face panel 14 in this order. Electron guns (only one is visible) 20 are disposed in the neck portion of the bulb 12. An electron beam 22 issuing from any one of the electron guns 20 is deflected by an electric field formed by a deflection yoke 24. The deflected beam 22 scans the fluorescent surface 16 via the shadow mask 18 and thereby displays a picture on the surface 16.
To enhance the basic performance of the CRT or display 10, i.e., contrast and luminance, a black matrix film (BM film hereinafter) , not shown, is formed on the inner surface of the face panel 14 integrally with the fluorescent surface 16. The BM film consists of red, green and blue light emitting pixels, and graphite or similar light-absorbing substance filling the spaces between the light emitting pixels. A metal back film, not shown, reflects incident light while being isolated from the fluorescent surface 16. The metal back film is implemented by an aluminum film.
As shown in FIG. 2, the shadow mask 18 is formed with a number of rectangular slots 26 for passing electron beams therethrough. The slots 26 each has a length L in the vertical direction V and a width S in the horizontal direction H. The slots 26 adjoining in the vertical direction V are spaced by bridge portions 28 while the slots 26 adjoining in the horizontal direction H are spaced by connecting portions 30. To form the slots 26, a particular rectangular resist pattern is formed on each of the front (facing the fluorescent surface 16) and the rear (facing the electron guns 20) of the material of the shadow mask 18, and then the material is etched. The resist patterns are rectangular, and each is long in the vertical direction V of the screen and short in the horizontal direction H of the same.
Specifically, as shown in FIG. 3, rectangular large holes 26L and rectangular small holes 26S are respectively formed in the front and rear of the shadow mask 18 by etching, constituting the desired slots 26. At the center of the shadow mask 18, the center CL of the large hole 26L and the center CS of the small hole 26S forming one slot 26 together with the hole 26L are coincident with each other. On the other hand, in slots towards the edge of the shadow mask, the centers CS of the other small holes 26S are deviated from the centers CL of the associated large holes 26L, with the center of the small holes skewed towards the center of the mask. The deviation of the center CS from the center CL is designed to sequentially increase from the center towards the outer horizontal edges of the shadow mask 18. Stated another way, the sectional configurations of the slots 26 are sequentially varied in order to increase the width S, FIG. 2, little by little toward the edges of the shadow mask 18. With this configuration, the mask 18 minimizes the quantity of, among exposing light having an angle of incidence, light directly incident to the slots 26 and undesirably reflected by the walls of the slots 26.
As stated above, during exposure for forming the fluorescent surface 16 on the inner surface of the face panel 14, the shadow mask 18 and face panel 14 are exposed in a pair so as to form the BM film. The exposure is influenced by the angle of the exposing light incident to the slots 26 and the width S of the slots 26. In light of this, basically all the slots 26 have their sections, as seen in the direction III--III of FIG. 2, sequentially varied in order to increase the width S little by little, as stated above.
However, the shadow mask 18 described above has the following problem left unsolved. As shown in FIG. 2, the mask 18 has a slot area 32 where the slots 26 are present and a non-slot area 34 where the slots 26 are absent. Consider the slots 26 located at the opposite edges of the area 32, particularly a slot 26a close to the corner of the mask 18. Because the slot 26a located at the edge of the area 32 adjoins the non-slot area 34, auxiliary exposing light for the slot 26a is available only from a slot 26b just inward of the slot 26a. It is therefore likely that the quantity of exposing light for the slot 26a is absolutely short. As a result, the BM film is apt to remain in the portions of the fluorescent surface 16 where stripes should be formed, as discussed earlier.
Thus, the problem with the conventional shadow mask is that an insufficient amount of auxiliary light for exposure is available for the slots adjoining the edges of the slot area of the mask, particularly the slots close to the corners of the mask. It is therefore likely that the quantity of light for such slots is absolutely short, resulting in the local omission of stripes on a fluorescent surface.